Bullet Sizes and Calibers Guide: Know Your Ammunition

When you’re getting into firearms, you are immediately tossed into a sea of numbers, letters, and statistics that can be very confusing.

This is especially true in terms of ammunition. This is because ammunition can have similarities in one dimension but be completely incompatible in terms of the guns they are made for. On the other hand, you can have multiple guns using the same ammunition type.

To clear things up, we’re going to go over the various gun calibers, the most common types of bullets, and what goes into classifying ammunition—starting with what caliber is and what that means for the bullet’s size.

We will only be covering ammunition for things like rifles, shotguns, and handguns. This excludes the larger ammunition that goes for things like autocannons or heavy machine guns, despite working similarly to smaller ammunition.

Bullets are made up of three major sections: the projectile/bullet, the case, and the propellant. These can be more refined, but this basic understanding is all we’ll need right now. When combined these are referred to as a cartridge or, confusingly, a bullet.

Let’s start with the basics of bullet sizes and calibers.

Caliber/Bullet Diameter

Bullet caliber and bullet diameter determine how wide the bullet or projectile is. This is measured in both Imperial/US measurements and Metric measurements. These systems measure the bullet’s diameter in fractions of an inch and millimeters, respectively. 

This measurement is determined by measuring the inside of the barrel. In rifled barrels, this is measured either from the high points of the rifling (referred to as lands) or the low parts of the rifling (referred to as grooves) and is the first number in the ammunition number sequence.

These two measuring methods can make some ammunition incompatible with some barrels despite having the same diameter. For example, 9x19mm Parabellum is not compatible with 9x18mm Makarov despite both having 9mm as their diameter.

This can be confusing as the bullet diameter or caliber is used to denote what type of ammunition a gun takes, especially as some bullets have multiple names.

Making it more troublesome is the measuring of shotgun caliber. Shotguns use the old measurement systems of cannons and are determined by how much lead projectiles the diameter of the barrel weigh. This usually is supposed to add up to a pound with modern shotguns.

This means a 12 gauge shotgun takes 12 lead balls, the diameter of the barrel to reach a pound, while a 20 gauge will take 20 lead balls, etc.

Bullet Length and Weight

These factors of bullets are usually not as crucial if you choose the correct caliber for your gun.

The bullet length and weight will impact the performance of the ammunition. However, only the bullet weight is noted on ammunition packaging after the ammunition caliber. 

Case Length

Case length is the second number in the ammunition numerical sequence for modern ammunition and determines how long the rest of the cartridge is. The case holds the propellant and also determines just how powerful some cartridges can be made.

This also makes bullets with the same diameter incompatible because they won’t fit into the rifle. For example, a 7.62x51mm and a 7.62.×39 are not interchangeable because their case lengths are different. The first round has a case length of 51 mm and the second has a case length of 39 mm.

A gun only has so much room for the whole cartridge and the case length is one of the major factors taking up that internal space.

Centerfire and Rimfire

These are the two ignition systems of the propellant and cover the major variations of cases.

Centerfire cartridges have a small disk in the base of the case. This is the primer and when it is hit by the gun’s firing pin it sets off the propellant. Centerfire cartridges are very reliable and are the most common style of case across multiple calibers.

The other ignition system is rimfire. This places the primer around the edge of the case’s base. This system is usually used in smaller caliber ammunition and was less reliable in the past. This is still partially true as more expensive versions of these calibers have better reliability, but lower-cost versions can have a higher failure chance.

To tell the two apart, just look for a small central disk on the bottom of the case. If there is no disk, it is a rimfire round. If there is, it is a centerfire round.

There are a lot of calibers out there, but thankfully most modern ammunition has settled down to being only a few dozen. This doesn’t include specialty rounds or more modern rounds. To keep things manageable, we will go over the most common rounds you’ll encounter.

We’ll break these down into three major groups: Handgun calibers, Rifle calibers, and Shotgun calibers.

Handgun Calibers

Handguns are either semi-automatic pistols or revolvers. Usually, revolver ammunition and semi-auto ammunition are not interchangeable, but this varies from gun to gun. So we’re going to start with one of the most common rounds available.

.22 Long Rifle

The .22 Long Rifle cartridge is the most common rimfire round in the world. Because of its low recoil and versatility, it is used in both handguns and small, light rifles.

It is usually used for target practice, recreational shooting, pest control, and the hunting of small game animals.

Due to its effectiveness on pests, it is recognized as being useful to the general public. This makes it one of the only rounds widely available to the civilian market, even in areas where firearms are heavily restricted.

The low pressure, light recoil, and low cost of this round make it ideal for training new shooters and for close-range precision shooting.

Revolver Calibers

Revolvers are some of the most easily recognizable handguns. The rotating cylinder clearly makes it different from semi-automatics. Since these were developed independently of semi-automatic designs, their ammunition differs as well.

Let’s get into some of the more common revolver ammunition.

.38 Special

The .38 Special cartridge was designed after the Spanish-American War to make up for the lack of performance by another cartridge, the .38 Long Colt.

The Special had a heavier projectile and a larger powder load. This produced a better effect than the Long Colt and set up the .38 Special for widespread use.

While it was originally developed to be a military service cartridge, it was never adopted. Instead, it became a popular self-defense and police load.

The round was made more powerful as the needs of the police evolved to counter bootleggers during Prohibition. This set up for another round, the .357 Magnum. 

However, .38 Special was made more and more powerful to match the performance of .357 Magnum after it was introduced due to negative press regarding the newer cartridge.

The more powerful loadings are still popular today as self-defense loads, particularly with snub-nosed revolvers.

.357 Magnum

This round was developed out of the push to strengthen the .38 Special. Because of this, guns chambered in .357 Magnum can use .38 Special ammunition, but .357 Magnum rounds cannot be used in .38 Specials. This is because the cartridge is designed to not fit the .38’s chamber as a safety measure.

This safety measure was added because the pressure of the Magnum cartridge was beyond the pressure tolerances of the .38’s chamber.

This was the first of the Magnum handgun cartridges. As such it was a very powerful round. Because of this, the recoil of the .357 Magnum is high. Despite the heavier recoil, this round is still a common option for those who want more power out of their handgun.

.44 Special

The .44 Special was designed in the early 1900s in conjunction with Smith & Wesson’s New Century revolver.

The .44 caliber projectile took inspiration from the popularity of .44 and .45 caliber projectiles favored in the American West and US Military service.

This resulted in a powerful, accurate cartridge that quickly was modified and adopted by the handloading community.

Today it is a lower recoiling alternative to use in guns chambered in .44 Magnum, similar to how .38 Special is used in .357 Magnum.

.44 Magnum

Introduced in the 1950s. The .44 Magnum was developed out of the .44 Special by handloaders. This resulted in the .44 Magnum having a longer case and more powder capacity than the .44 Special but with most other measurements being the same.

The higher velocities made the round effective but severely impacted the ability to fire rapidly due to recoil. The higher recoil also increased shooter fatigue and the stress on the shooter’s hands.

This round was made popular not from its good performance at range but its appearance in the Clint Eastwood Dirty Harry movie series.

Semi-Automatic Calibers

Semi-automatics have come to eclipse revolvers in most roles today. These roles include civilian, military, and police uses. Mostly this comes down to self-defense. This narrowing of use has made semi-automatic calibers more refined and standardized than other ammunition.

While there are dozens of semi-automatic rounds on the market you’ll most likely encounter four major variations: 9 mm, .45 ACP, .40 S&W, and .380 ACP

9 mm

The 9×19 Parabellum cartridge, sometimes referred to as 9mm Luger or just 9mm, is one of the most popular semi-automatic handgun rounds on the market today. It is the primary option for self-defense in the US.

This round is also used in multiple shooting competitions and is prized for its low recoil and performance. The 9 mm round allows for more bullets to be carried in a pistol while still providing acceptable performance with modern hollow point ammunition.

The comparatively lower recoil also allows for better control of the pistol, quicker follow-up shots, and better accuracy.

This round was first developed in 1901 and has had over 100 years of continual development.

.45 ACP

The next most popular semi-automatic handgun cartridge, the .45 ACP was the round of choice for self-defense after both World Wars.

Developed for the US Military in 1904, this round was eventually used in the famous 1911 pistol. Because of its prevalence and the experience former soldiers had with the round, it grew as a civilian cartridge through the 1950s up until the 1980s when the round began to be replaced with 9 mm in more modern handguns.

The heavier bullets and wide variety of load types, still make the .45 ACP cartridge a popular option today, but these benefits come at the cost of higher recoil and a lower capacity in handguns than 9 mm options.

This cartridge is still usable but should be used with an understanding of its limitations. 

.40 S&W

The third most popular handgun round in the defensive triangle (9mm, .45 ACP, .40 S&W), the .40 S&W is the result of a lower-powered version of 10 mm.

While 10 mm has become more obscure, its descendent grew to some popularity. The .40 S&W cartridge is known for snappy recoil, low capacity, and moderate control.

In practice, it takes the negatives found in .45 ACP guns and puts them in the small frame of a 9 mm handgun. This resulted in a round that was quickly fazed out of use by most police agencies after it was adopted.

While it is still a commonly found round, this is mostly because of its ties to law enforcement and the subsequent police trade-in pistols.

.380 ACP

The .380 ACP cartridge was developed in 1908 and was designed for the Colt 1908 pocket pistol. This small cartridge is still used today as a self-defense round.

The round was adopted across the globe and was comparatively more powerful than .32 ACP and .38 ACP. Despite this, it was still not as powerful as 9 mm.

Despite it being inferior to 9mm it is still a popular option for self-defense. This is because it provides acceptable performance with the right loading and is easy to control.

Rifle Calibers

Rifles provide most of the power when it comes to ammunition. The most popular versions of rifle ammunition are mainly connected to development and adoption by various militaries. However, not all military adopted rounds have become popular.

Here are some of the most common still in use today.

.30-06

This round was adopted by the US military in 1906, hence the “06” at the end of its name. This round was used as the standard rifle cartridge in World War I and World War II.

The large number of surplus firearms chambered in this cartridge and its phenomenal performance for the time made this round popular with hunters. It is still used today because of its long-range ability and good performance on large game animals.

The .30-06 cartridge is also heavily handloaded to meet the desires of the individual shooter.

7.62×51/.308

These rounds are derived from .30-06 as a result of advancements in propellant technology. Introduced in the 1950s, the 7.62×51 round was adopted as the official battle rifle cartridge of NATO and the US Military. At the same time, the round was released to the civilian market as the .308 Winchester cartridge.

Because of this, the refinement of the rounds has made them very similar in performance. They are roughly interchangeable in modern firearms with only slight variations in the pressures they produce.

These are both popular hunting and precision rounds, filling a similar role as .30-06 but with a smaller case length.

5.56×45/5.56 NATO/.223 Remington

The first official intermediate cartridge adopted by the US, the 5.56×45/.223 Remington round bridges the gap in power between pistol and full-powered rifle rounds. This means better recoil, higher capacities, and good performance within a maximum of 500 meters.

This round was developed out of the experiences of the US in both World Wars and the combat in both Korea and Vietnam.

The 5.56 cartridge still offered rifle lethality at most normal combat distances but would fit in a lighter, more controllable firearm. This resulted in the round being adopted by NATO and other countries besides the US.

This round has also seen adoption as a self-defense/home-defense round by civilians in the US, especially with more modern ammunition options.

7.62×39

One of the first intermediate cartridges developed, this round was adopted by Russia after the Second World War. Commonly known as the AK’s bullet, 7.62×39 indirectly inspired the development of the 5.56 cartridge.

However, it has become widely used in both military and civilian circles. It provides better performance on barriers and dense game animals than 5.56 but has more risk of over-penetration, resulting in hitting unintended targets.

Shotguns Gauges

Shotguns use shells and are referred to by gauge rather than caliber. This makes it somewhat annoying to compare shotguns to other guns but makes it easier to select your ammunition as long as you match the gauges correctly.

12 Gauge

The 12 gauge family of shells is one of the most popular shell types in the United States. It has been the preferred shotgun gauge since the early 1900s, as such there are dozens of different loads for 12 gauge.

This means 12 gauge offers the most versatility in the world of firearms as there are shells for combat, hunting, and law enforcement uses.

However, this versatility is offset by stout recoil with some loads and lower-capacity firearms.

20 gauge

The 20 gauge shotgun is the next most popular shell type. This option has lower recoil but less versatility than 12 gauge and is popular outside the United States.

The lower variation of loads does not make the 20 gauge an un-viable option, but it does limit its application. This round is perfect for those who do not want to deal with a lot of recoil but still want some of the variety that using a shotgun offers

The projectiles fired by firearms come in many styles. Here are some of the most common.

Ball Ammunition

Ball ammunition can have a small layer of copper around it or not. This is the most basic type of ammunition as it is the basis of almost all ammunition.

These solid projectiles are usually standardized as the baseline in military firearms and relegated to training ammunition in civilian circles.

Full Metal Jacket

Full Metal Jacket or FMJ is the most common variation of ball ammunition. You can find any common pistol or rifle round in FMJ.

This ammunition features a basic ball ammunition projectile with a layer of copper jacketing around the entire projectile.

Full Metal Jacketing was introduced to ammunition after firearms started using better propellants. The velocities that bullets were going at would destroy the solid lead projectile, but adding a layer of sturdier metal over them solved the problem of the bullet disintegrating as it was being fired.

Hollow Point

Hollow point ammunition features a small cavity in the projectile that allows the projectile to open like a flower. This increases the wounding capacity of the round and its effectiveness. Most modern self-defense rounds are hollow points.

While hollow points do create bigger holes in the target, it does not allow the round to deal with barriers as well as FMJ does. This is usually not an issue in most self-defense situations.

Soft Point

Soft point ammunition is usually found on revolver rounds or rifle rounds used for hunting. This bullet design leaves some of the lead exposed at the tip of the bullet. This is done to cause the bullet to deform on impact creating a hollow-point-like expansion while providing better aerodynamics.

Bonded Hollow Point

These types of rounds feature the hollow point cavity but are filled with polymer. This is to provide similar performance to a soft point round but is usually used in areas that outlaw hollow point ammunition

Bonded hollow points are great alternatives to both hollow points and soft points in rifles and are popular with long-range hunting rounds.

Slugs

The single large projectiles of shotguns are referred to as slugs. This helps make the shotgun similar to a rifle and lessen the chance of accidental hits because the shotgun is only putting out a single projectile.

Slugs are a very popular hunting option since they deal with dense game animals well, and are usually legal in more areas than rifles are.

Shot

Shotguns are known for using small projectiles referred to as shot. They come in many different sizes and each has a different use.

Some are better for bigger game, others are great for birds, and others are more for law enforcement use. However, the most popular shot options are variations of buckshot.

All of these options come down to one question. Which one should you use?

In the modern world, there are many great options available for whatever task you need. But it all comes down to how well you can control the firearm you are using.

This means rounds like 9 mm, .38 Special, and .44 Special are going to be the better options for handguns.

For rifles, 5.56×45 and 7.62×39 are more controllable than other rifle options, but for longer range tasks 7.62×51/.308 is the better option.

Selecting a good shotgun round requires more thought. If you want something that is going to put a lot of shot on target or one powerful projectile, 12 gauge is going to be the option you choose. But 20 gauge offers more control.

All of this comes down to what is available in your area and what is in stock consistently? If your area doesn’t support any of these options it would be better to select the rounds that are available locally instead of only relying on ordering your ammunition from the internet.